1. Filed of Invention
This invention relates to communication circuits, particularly to transceiver circuits.
2. Brief Description of Related Art
The generation of a quadrature frequency signal in addition to an in-phase signal is widely used in modem direct frequency conversion (homodyne) circuits. The homodyne circuits increase image rejection and avoid the expensive intermediate frequency bandpass filters (where SAW filers are commonly used). However, the homodyne circuit requires both an in-phase signal and a quadrature phase signal to beat with the incoming radio frequency signal so that the beat frequency output is complete.
In a paper given at the 2003 IASTED Circuit, Signals and System Conference, a paper by Ming-Hao Sun and H. C. Lin, entitled: xe2x80x9cAutomatic Quadrature Phase Generatorxe2x80x9d was presented. The circuit is shown in FIG. 1a, where a voltage-controlled oscillator (VCO) is used in phase-locked loop, which includes a phase comparator (PC) which is a multiplier, a low-pass filter (LP). The input signal (VI) to the phase comparator is at quadrature phase with the feedback signal from the VCO and the feedback signal is derived as the quadrature phase signal (VQ). However, in a typical frequency synthesizer, the VCO frequency is divided by a frequency divider to compare with the reference as shown in FIG. 1b. The drawbacks of this circuit in FIG. 1a are: (1) a VCO is required, and (2) the quadrature phase signal VQ is that of the divided frequency, not the quadrature phase frequency of the VCO, while the desired quadrature phase signal is usually that of the VCO.
The object of this paper is to generate a quadrature phase signal from an in-phase signal without using a VCO. Another object of this paper is to generate a wideband quadrature phase frequency signal. Still another object is to generate equal amplitude in-phase and quadrature phase signals.
These objects are achieved by using an analog multiplier. One multiplicand is the in-phase input signal VI, and the output of the multiplier is set to zero. Then a quadrature phase signal is derived from the other multiplicand. When a differential pair multiplier is used as the multiplier and VI is applied to control the current source, a quadrature phase signal appears at the input of the differential pair. The ac output is set to zero by short-circuiting the output directly to the positive power supply. Other kinds of analog multipliers may also be used.